Carl von der linde



NITED STATES ATE/NT rrrcn.

PROCESS OF DISTILL-ING AC'ET-IC ACID.

SPECIFICATION forming part of Letters PatentN'o. 596,043, dated December 21, 1897.

Application filed June 28,1895. Serial No. 554,348. (No specimens.) Patented in Germany November 29,1894, No. 92,418; in England February 18, 1895,1510. 3,477; in Hungary June 5,1895,N0.41,985; in France June 9, 1895, No. 246,056 in Belgium July 14, 1895, No. 122,604, and in Austria August 9,1'895,No.45/21831.

Great Britain, No. 3,477, dated February 18, l 1895; France, No. 246,056, dated June 9,1895; I

Belgium, No. 122,504, dated July 14, 1895; Austria, No. 45/2,831, dated August 9, 1895,

.and Hungary, No. 41,985, dated June 5,1895,)

which improvements are fully set forth in the I following specification.

The object of the present invention is an improved process of obtaining acetic acid by means of a pyroacetate and a mineral acid,

which process is carried out under special conditions, as will be hereinafter explained.

As heretofore practiced the manufacture of acetic acid on an industrial scale has been carried on principally by two methods: first,

employed, both at normal pressure. In each of these two methods the first step was to mix i in a cast-iron vessel heated upon an open fire a certain quantity of pyr'oacetate with a certain predetermined quantity of acid, the latter generally exceeding by ten to fifteen per cent. the quantity theoretically necessary. Distillation was then produced by heating, at first slowly, then toward the end of the operation increasing the temperature, in order to drive out as much as possible all the acetic acid which remains in the vessel. In this operation (particularly when sulfuric acid was used) a temperature of 300 to 400 centigrade was attained. The objections to these processes are as follows:

(1.) Those attending the sulfuric-acid process.-From the moment the temperature reaches 80 centigrade the hot sulfuric acid attacks the tarry matters, which are strong reducing agents and which are always present in certain proportions in pyrolignites. It transformed a portion of these substances into volatile colored matters, which imparted a bad odor to and discolored the distilled acetic acid. On the other hand, the sulfuric acid became reduced to sulfurousacid,which dissolved in great part in the acetic acid,

while the remainder which escaped was 'offensive and dangerous to the workmen. The

sulfate of lime, of which the residue is com-' posed, becoming mixed with the tarry substances of the pyrolignites, formed with them a pasty mass difficult to manipulate, requiring a prolonged heating at a very high temperature to remove from it the aoidit retained. The influence of this high temperature had the effect of intensifying the result above stated and caused decomposition of a certain quantity 'of acetic acid already formed. The acetate of lime itself was in ,partdecomposed to form acetone, which also contributed to give the distillate a very bad odor. The wear and destruction of the boilers exposed to the fire were very rapid, and notwithstanding their thickness (forty millimeters) they were corroded and burned out in from six to eight months. It was also practically impossible to make use of the brown pyrolignite, the price of which is less than that of the pyrolignite of the gray acetate, but which contains too many reducing agents to admit of its successful employment with sulfuric acid. To sum up, therefore, there is, first, a great reduction of the quantity of sulfuric'acid and consequent loss of 80,11 necessitating an excess thereof from the start; second, objections from a sanitary point of View; third, formation of volatile pyroligneous products contaminating the acid; fourth, destruction of a certain quantity of acetate by conversion into acetone fifth, loss of time on account of the necessity of prolonging the operation; sixth, excessive cost of fuel and powder; seventh, rapid destruction of the vessel; eighth, production of an acid discolored, impure, and charged with sulfurous acid, all of which necessitated repeated rectification over oxidizing agents to effect purification.

(2.) Objections to the hydrochloric-acedproccss.-The process has been almost exclusively employed industrially because it permitted the use of the brown acetate and had the advantage of avoiding formation of sulfurous acid. The objections, however, are as follows: Equally with the former process the hydrochloric-acid process required a very high temperature, which is attended, first, by formation of acetone and consequent loss of acetate; second, by formation of highlycolored empyreumatic products which are strongly odoriferous third, by rapid destruction of boilers; fourth, by long duration of operation; fifth, by production of a colored and impure acid requiring one or more rectifications before it becomes a commercial article; sixth, by a certain quantity of hydrochloric acid being distilled over with the acetic acid, thus degrading the quality of the latter; seventh, by the impossibility of obtaining acetic acid of higher degree of concentration than forty to forty-five per cent. eighth, and principally, by the increased expense of using hydrochloric acid, which is far more costly than sulfuric acid.

By the present process the foregoing obj ections, which attend both the industrial methods referred to above, are obviated, and, moreover, great advantage is secured in respect of the quality and degree of concentration of the product and in respect of rapidity of production.

This process is based on the principle of lowering the temperature during the reaction of the acid on the acetate and during the distillation of the formed acetic acid. This lowering of temperature is effected by maintaining in the interior of the apparatus as nearly a perfect vacuum as possible.

The operation in accordance with my invention is effected in the following manner: take, for example, pyrolignite of lime and sulfuric acid, to which my invention is more specially applicable. In a cast-iron boiler provided with a mechanical agitator, within which a maximum vacuum can be maintained by means of a powerful exhaust-pump, I mix a charge of pyrolignite with the quantity of sulfuric acid theoretically necessary to effect the reaction. The following will occur: The acetic acid boils in vacuo at about 26 centigrade- 2'. a, far below the temperature at which sulfuric acid is decomposed by the tarry matters present. The heat produced by the reaction is thus employed in the vaporization of the acetic acid without raising the temperature. The sulfuric acid therefore cannot become sufiiciently heated to attack the tarry substances. There is consequently no formation of sulfurous acid, nor empyreumatic, nor of tarry products, nor is there any formation of acetone or loss of sulfuric acid or of the acetate. The entire operation takes place in the closed vessel without a trace of acetic acid being lost and without any escape of non-condensed vapors into the room where the workmen are employed. If the vacuum should diminish, the temperature of ebullition of acetic acid would rise, and in consequence the objections attending the old processes would instantly appear. It is therefore of the utmost importance to maintain the vacuum as high as possible. It should never fall below six hundred millimeters of the mercury column, equal to about one hundred and fifty millimeters absolute or barometric pressure. It will be understood that an exhaust-pump such as has been used to produce evacuation of the sulfurous gas that may have been formed could not attain the object in view.

When the heat produced by the reaction has been totally absorbed by vaporization of the acetic acid, (which only happens after seventy to seventy-five per cent. of the product is distilled,) heat may be applied moderately, and for this purpose steam is the preferred agent for the reason that it admits of regulation and of easy supervision. By avoiding the use of an open fire the destruction of the boilers is also avoided.

The acid distils with great rapidity, which reduces the length of the operation more than half and increases the output of the apparatus, while diminishing the labor of the attendants.

As to the product, it is of a perfect limpidity and absolutely without coloration or nauseating odor. Its concentration is extremely high (from sixty-five to eighty-five per cent., according to the sulfuric acid used) and does not vary during the whole duration of theprocess. The degree of vacuum exercises no appreciable influence on the concentration.

By thus employing the maximum vacuum the following advantageous results are obtained: first, the employment of a smaller quantity of the material; second, the production of a purer and more concentrated acid requirin g no subsequent rectification third, the employment of the brown acetate with sulfuric acid, thereby effecting a great economy in the materials; fourth, economy in time; fifth, a large increase in the output of the plant.

Although I have confined myself to pointing out the most prominent results of this improved process, I would lay stress upon the importance to be attached to the difference between the distillation in vacuo of the crude acid obtained by the old processes and the direct production of the pure acid in 'vacuo during the decomposition and during the distillation. This double action in vacuo is indispensable and is characteristic of my process. The two operations-decompositionand distillation-take place simultaneously in the same apparatus, and unless this is observed it is impossible to obtain the described results.

Having now particularly described my invention, what I claim as new, and desire to secure by Letters Patent, is,

1. The process of directly obtaining pure and colorless acetic acid free from sulfurous acid, acetone and empyreumatic products, by the reaction of a pyroacetate and mineral acid, the simultaneous decomposition of the mixture and distillation of acetic acid being carried on in a closed vessel within which a maximum Vacu um is maintained (not less than six hundred millimeters of the mercury column) at a temperature below that at which the mineral acid attacks the tarry matters, and below that at which acetate and the formed acetic acid decompose, substantially as described.

2. The process of directly obtaining pure acetic acid by the reaction of sulfuric acid upon a pyroacetate, the simultaneous decomposition of the mixture and distillation of the acetic acid being effected in a closed vessel in which a maximum vacuum such as specified is maintained, the operation being conducted at a lower temperature than that at which the sulfuric acid attacks-the tarry matters of the I 5 pyroacetate, substantially as described.

In Witness whereof I have hereunto set my hand in presence of two Witnesses.

CARL VON DER LINDE. Witnesses:

T. BRUREN, EMIL W. ANSLEBEN. 

